When a cuff wrapped around a human body part is pressurized to a certain level with a pump and is then gradually depressurized, the internal pressure of the cuff cyclically changes in synchronism with his pulse. This cyclic change in the internal pressure of the cuff is called as pulse wave, and the oscillometric processes generally refer to a number of different processes for determining blood pressure values according to analysis of the wave form of the pulse wave.
Specifically, when the cuff pressure is increased beyond an expected systolic pressure level and is then gradually decreased, the amplitude of the pulse wave is initially small and gradually increases as the cuff pressure declines. As the cuff pressure reaches near an average blood pressure level, the amplitude of the pulse wave attains a maximum level, and then decreases as the cuff pressure declines further. According one of the oscillometric processes, a systolic pressure and a diastolic pressure are given as the cuff pressures at which the amplitude of the pulse wave becomes equal to certain fractions (X % and Y %) of the maximum amplitude of the pulse wave (refer to FIG. 7). According to another oscillometric process, a systolic pressure and a diastolic pressure are determined as the cuff pressures at which the increase rate of the amplitude of the pulse wave sharply increases and sharply decreases (refer to FIG. 8).
However, the behavior of the amplitude of pulse wave can vary a great deal depending on each individual. In the case of one of the aforementioned oscillometric processes, the true systolic pressure and the true diastolic pressure may not be prescribed fractions of the maximum amplitude of pulse wave. In the case of the other oscillometric process, the points of sharp changes in the change rate of the amplitude of the pulse wave are in many cases unclear. For these reasons, significant measurement errors have been encountered in many cases.
In such oscillometric processes for electronically measuring blood pressure values, the cuff must be pressurized at least beyond a conceivable systolic blood pressure value. When pressurization is not sufficient, an accurate measurement is impossible. Because such insufficiency of pressurization is not discovered until the process of measuring blood pressure has progressed a certain extent, the cuff must be repressurized upon discovery of such an insufficiency and the time required for blood pressure measurement is inevitably prolonged to a considerable extent. Conversely, if the cuff is pressurized excessively, not only the time required for blood pressure measurement will increase because an earlier part of gradual depressurization merely wastes time but also the discomfort of the person wearing the cuff is unduly prolonged.